The present invention relates to drywall compound sanding discs of the type comprising a polymeric foam disc and an abrasive disc having a diameter larger than that of the foam disc that is adhered to one major surface of the foam disc and includes a layer of abrasive material on its surface opposite the foam disc by which drywall compound may be sanded when the sanding disc is rotated by a drive motor while its abrasive surface is pressed against the drywall compound.
Known in the art is a drywall compound sanding disc comprising a polymeric foam disc and an abrasive disc including a layer of abrasive material defining one major surface of the abrasive disc. The abrasive disc has a diameter larger than that of the foam disc and has its surface opposite the abrasive material co-axially adhered to one major surface of the foam disc. Such sanding discs may be used to sand drywall compound (i.e., the compound used to fill and cover the joints between the edges of adjacent sheets of drywall that are used to form the inner surfaces of rooms during the construction of houses, commercial buildings and the like) by rotating the sanding disc using a drive motor while the abrasive on the sanding disc is pressed against the drywall compound. One such drywall compound sanding disc is commercially available from Porter Cable Co., Professional Power Tools, Jackson, Tenn., and is adapted to be driven by the model 7800 Power Drywall compound sanding Tool also commercially available from Porter Cable, Professional Power Tools. While such drywall compound sanding discs driven by that tool can be used to effectively smooth dry wall compound, they present several problems. The major surface of the sanding disc defined by the layer of abrasive material is all generally planar so that the portion of the sanding disc adjacent to its peripheral edge has a tendency to gouge drywall compound to be smoothed if it is pressed against that dry wall compound with much of an angle between the surfaces of the drywall and the major surface of the sanding disc. Also, that peripheral edge of the sanding disc has a tendency to catch on projections above the surface of the drywall compound (e.g., electrical outlet boxes) which can tear the sanding disc and make it unsuitable for further use. Additionally, the portion of the abrasive disc on such a sanding disc adjacent to its peripheral edge that projects past the foam disc has a tendency to be or become rippled or curled, which rippling or curling appears to be accentuated by humidity fluctuations. Such rippling or curling of that edge portion exacerbates the problems of gouging and catching on projections mentioned above.
The present invention provides a sanding disc generally of the type described above that comprises a polymeric foam disc and an abrasive disc having a diameter larger than that of the foam disc that is adhered to one major surface of the foam disc and includes a layer of abrasive material (e.g., 60 to 180 grit) along its major surface opposite the foam disc that defines a first major surface of the abrasive disc by which a substrate such as drywall compound may be sanded when the sanding disc is rotated by a drive motor while its first major surface is pressed against the substrate. The sanding disc according to the present invention is improved so that it has significantly less tendency to gouge a substrate such as drywall compound to be smoothed even if it is pressed against that substrate with an angle between the surfaces of the substrate and the first major surface of the sanding disc, it has significantly less tendency to catch and be torn on projections above the surface of the substrate, and it restricts rippling or curling of the portion of the abrasive disc adjacent its peripheral edge.
The abrasive disc in the sanding disc according to the present invention includes a circular central portion, with the portion of the first major surface along that central portion being generally planar; and further includes an annular peripheral portion extending from that central portion to the peripheral surface of the abrasive disc. The portion of the first major surface along that annular peripheral portion of the abrasive disc is generally cylindrically convex and the portion of the first major surface along the circular central portion is generally tangent to the portion of the first major surface along the adjacent part of the annular peripheral portion so that the curvature of the annular peripheral portion positions the peripheral surface of the abrasive disc in a plane passing through the foam disc.
This generally cylindrically convex or inversely cupped portion of the first major surface along the annular peripheral portion of the abrasive disc has less tendency to gouge a substrate of drywall compound to be sanded as it is brought into contact with that drywall compound and has less tendency to catch and be torn on projections above the drywall compound than does the planer edge portion of the prior art drywall compound sanding disc described above. Also, this generally cylindrically convex or inversely cupped shape of the annular peripheral portion of the abrasive disc according to the present invention restricts the rippling or curling of the abrasive disc adjacent its peripheral edge which occurs in the prior art drywall compound sanding disc described above.
The portion of the first major surface on the generally cylindrically convex annular peripheral portion of the abrasive disc can, at each location around the annular peripheral portion, be generally circular around an axis (called a peripheral portion axis herein) in a plane parallel to and passing through the axis of the abrasive disc and that location. The radius of the portion of the first major surface on the annular peripheral portion of the abrasive disc around the peripheral portion axis at each location around the annular peripheral portion can be the same radius from the range of about 2 to 7 inches or 5 to 18 centimeters, with a radius of about 6 inches or 15 centimeters having been found to be more effective than either a larger or a smaller radius.
A novel method for forming the sanding disc according to the present invention can include method steps used for forming the prior art drywall compound sanding disc described above, which method steps include (1) providing the polymeric foam disc with the structure described above, and (2) providing an abrasive disc with the structure described above except that it has planar major surfaces, (3) providing a sheet of hot melt adhesive; (4) positioning the sheet of hot melt adhesive between a major surface of the abrasive disc opposite its layer of abrasive material and one major surface of the foam disc with the discs coaxial; and (5) heating and pressing together the positioned abrasive disc, sheet of hot melt adhesive, and foam disc to soften the sheet of hot melt adhesive and cause it to adhere the abrasive and foam discs together when the sanding disc cools. Additionally, the method according to the present invention includes the steps of (6) heating an originally generally planar annular peripheral portion of the abrasive disc extending from a planar central portion of the abrasive disc to the peripheral surface of the abrasive disc, and (7) reshaping the heated originally generally planar annular peripheral portion of the abrasive disc so that, after the abrasive disc cools, the portion of the first major surface defined by the layer of abrasive along that annular peripheral portion of the abrasive disc is generally cylindrically convex with the portion of the first major surface along the circular planar central portion being generally tangent to the adjacent part of the first major surface along the annular peripheral portion. Such generally cylindrically convex curvature of the first major surface along the annular peripheral portion positions the peripheral surface of the abrasive disc in a plane passing through the foam disc.
We are not sure why that the generally cylindrically convex shape of the first major surface along the annular peripheral portion is retained after the abrasive disc cools, but speculate that during the heating, shaping, and cooling of the abrasive disc (e.g., heating at 375 degrees Fahrenheit or 190 degrees Centigrade for 30 seconds while shaping the abrasive disc and then cooling at ambient temperature) latex in the cloth backing of the abrasive disc is softened when heated and again solidifies when cooled to retain the shape of the backing in the manner that starch shapes cloth when the cloth is ironed, and/or that micro-cracks formed when the annular peripheral portion is shaped in a resin layer attaching the abrasive to the backing of the abrasive disc are annealed by the heat applied to the disc; however other factors may also be involved.
While the steps of (6) heating and (7) reshaping the annular peripheral portion of the abrasive disc could be done prior to the step (5) of heating and pressing together the positioned abrasive disc, sheet of hot melt adhesive and foam disc; it is very convenient to perform those steps (6) and (7) during that step (5) by using a specially shaped surface that contacts the abrasive disc on a part of a press by which step (5) is performed.